The present invention relates to communications in computer networks. More specifically, it relates to transmission techniques used in digital subscriber loop modem technology.
A. Description of the Related Art
1. Asymmetric Digital Subscriber Lines
Asymmetric Digital Subscriber Line (ADSL) is a communication system that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices, as opposed to multi-point, where numerous devices share the same physical medium.
ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by the ADSL modemsxe2x80x94the POTS channel operates in the standard POTS frequency range and is processed by standard POTS devices after being split from the ADSL signal.
The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation.
2. Discrete Multi-Tone Modulation
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated using a type of Quadrature Amplitude Modulation (QAM). Alternative types of modulation include Multiple Phase Shift Keying (MPSK), including BPSK and QPSK, and Differential Phase Shift Keying (DPSK). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers. The symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the xe2x80x9c binxe2x80x9d associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithm called Inverse Fast Fourier Transforms (IFFT).
The symbol period is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band.
Because the symbol time is relatively long and follows a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each subcarrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 subcarriers, each with a 4 kHz bandwidth. Each sub-carrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the sub-channels.
The sub-channels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a sub-channel, the data in each sub-channel can be demodulated with a small amount of interference from the other sub-channels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension at the end of each symbol, in order to maintain orthogonality.
The DSL modulation technique described above is a constant carrier and is not designed for multi-point media. The architects of DSL services assume an xe2x80x9calways availablexe2x80x9d model. This translates into xe2x80x9calways onxe2x80x9d service, which leads designers to select constant carrier modulationxe2x80x94that is, constant line power, regardless of data traffic. Most of the time, DSL transceivers send idle packets. This maximizes energy consumption, rfi, crosstalk, etc. without corresponding benefit. In comparison, multi-point systems use switched carrier methods to accommodate multiple transmitters on the shared medium. Transmitters turn on carriers when required and are otherwise silent. Switched carrier mode tends to minimize energy consumption, as well as radio frequency interference and cross talk.
3. Frequency Domain Equalization
In standard DMT modulation, each N-sample encoded symbol is prefixed with a cyclic extension to allow signal recovery using the cyclic convolution property of the discrete Fourier transform (DFT). Of course, the extension may be appended to the end of the signal as well. If the length of the cyclic prefix, L, is greater than or equal to the length of the impulse response, the linear convolution of the transmitted signal with the channel becomes equivalent to circular convolution (disregarding the prefix). The frequency indexed DFT output sub-symbols are merely scaled in magnitude and rotated in phase from their respective encoded values by the circular convolution. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, sub-channel isolation is achieved. Thus, the original symbols can then be recovered by transforming the received time domain signal to the frequency domain using the DFT, and performing equalization using a bank of single tap frequency domain equalizer (FEQ) filters. The FEQ effectively deconvolves (circularly) the signal from the transmission channel response. This normalizes the DFT coefficients allowing uniform QAM decoding.
Such an FEQ is shown in FIG. 1. The FFT calculator 20 accepts received time domain signals from line 10, and converts them to frequency domain representations of the symbols. Each frequency bin (or output) of the FFT 20 corresponds to the magnitude and phase of the carrier at the corresponding frequency. In FIG. 1, each bin therefore contains a separate symbol value X(i) for the ith carrier. The frequency domain equalizer FEQ 40 then operates on each of the FFT 20 outputs with a single-tap filter to generate the equalized symbol values Xxe2x80x2(i). The FEQ 40 inverts the residual frequency response of the effective channel by a single complex multiplication. The FEQ outputs are then decoded by a slicer, or data decision device (not shown). The FEQ taps can be updated, and can make use of the slicer output. That is, the FEQ taps may be updated so as to minimize the error between the FEQ output and the slicer output. This is commonly referred to as decision feedback equalization, or decision-directed adaptation.
Equalization in packet based DSL modulation has certain advantages over this prior art. In the prior art, some channels contain nulls or near nulls, and are difficult to equalize. This is especially true when the equalizer solution is supposed to remain stable for long periods. These problematic spectral features are less significant when the equalizer solution need only be designed to last for a short period. The transmitter gating function is a time domain pulse that looks like a sinc function in the frequency domain. The convolution of this sinc function with the channel yields an apparent channel with shallower nulls. The equalizer faces a less severe problem. The packets can be constructed for some maximum length that makes short term equalization easy and timing drift tolerable. Of course, there is a preamble penalty in a packet based systemxe2x80x94there is at least some training preamble in every packet, which reduces the peak data throughput somewhat, though the statistical crosstalk savings may more than compensate for it.
4. Timing Recovery
Also shown in FIG. 1 is a clock recovery and control circuit 30. The clock recovery circuit 30 analyzes the pilot tone that is embedded in the transmitted DMT signal in ADSL communication systems. Timing recovery normally increases receiver hardware and software complexity and may require some spectral allocation, depending on type of modulation or coding.
A typical hardware solution is shown in FIG. 2. The clock recovery components are indicated with dashed lines. Control words from a clock recovery algorithm running in a DSP 10 are converted to voltage levels by a digital-to-analog converter (DAC) 12 which controls the receive sampling rate of an ADC 14 through a voltage-controlled oscillator (VCO) 16. With a pure software timing recovery solution, the DAC 12 and VCO 16 (marked in dotted line) and any associated circuitry can be eliminated. A crystal at the nominal frequency would provide the ADC 14 sample clock.
Timing information may also be recovered using software techniques. One known method of implementing software clock recovery is to digitally resample the received signal at the transmitter""s clock rate by interpolating the received samples. FIG. 3 is block diagram showing an input signal being applied to an interpolator 12, the output of which is applied to a clock recovery algorithm 22 executing on a processor. The interpolation may be performed by clock recovery.
This is a general method and may be used to recover clock for any type of synchronous modulation but may differ in the way the transmit clock is extracted from the received signal. For DMT, one of the frequency bins output from a fast Fourier transform (FFT) function is usually dedicated as a pilot tone. The clock recovery algorithm executing in a processor includes a filter to isolate the pilot tone and logic to estimate the clock offset between transmitter and receiver and to control the resampling rate in an interpolator.
The interpolation stage can be implemented a number of ways, but it generally consists of integrally interpolating receive samples to a rate (k) several times the nominal rate and then fractionally interpolating between two or more high rate samples using polynomial interpolation. Decimation to the final rate is accomplished by skipping over (not computing) samples and by computing only those samples needed to fractionally interpolate to the final rate. The interpolation rate (k) and the order of the polynomial used in the fractional interpolation can be traded off for a particular implementation in order to minimize complexity and provide tolerable interpolation error. Because this method can require tens of processor cycles for each interpolated sample it is not preferred for modems operating at high sample rates.
Because ADSL and other DMT modems are high-speed high bandwidth communication devices, they generally rely heavily on hardware solutions timing recovery solutions. As modem technologies mature, often the most important differentiator between one modem manufacture and the next is cost. One way to reduce cost is to reduce the hardware complexity of a modem. Cost savings can be significant for very high-speed modems with sample rates in the range of hundreds of kilohertz to several megahertz because of the premium placed on high-speed components. Of course, a software solution takes processor resources, which also has a cost, but processors tend to have steep development curves and newer versions are regularly introduced with increased capability and lower cost.
In contrast to the prior art, a packet-based system may not require any tracking timing recovery scheme, depending on the accuracy of the timing references and the packet length. In some cases, expensive hardware functions, like the VCO, can be eliminated altogether. Additionally, the spectral allocation, if any, can be reduced or eliminated.
In accordance with preferred embodiments of the present invention, some of the problems associated with constant carrier DSL modulation are overcome. The subject invention defines the details of packet based DSL modulation. One aspect of the invention includes a method for minimizing energy consumption, radio frequency interference, and ensemble crosstalk generation by specifying a duty cycle for bursted carrier data traffic. It is also a purpose of this invention to simplify equalization and timing recovery while also providing opportunistic echo canceller training.
The foregoing and other features and advantages of a preferred embodiment of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.